Aluminum oxynitride of cubic spinel type (γ-AlON) is Al2O3—AlN having a structure of stable single-phase and cubic solid solution, and belongs to the transparent polycrystalline ceramic. It has a hardness of 17.7 GPa and a strength of 380 MPa, which is second to the single crystal sapphire only. As compared with the single crystal, γ-AlON has better moldability and is easier to be prepared into components with large size and different shapes. γ-AlON has a melting point higher than that of alumina, and thus possesses good high temperature resistance, thermal stability and corrosion resistance, thereby being an ideal structural ceramic and refractory material. γ-AlON has good transmittancy in the range of UV-visible and mid-infrared light (transmittance >80% at a wavelength of 0.2 to 5 μm), and, therefore, has great potential for applications of infrared high temperature-resistant windows, bulletproof armor materials, and fairings.
Conventionally, the synthesis of aluminum oxynitride powder is mainly performed by solid-state reaction as well as carbothermic reduction and nitridation. The solid phase reaction employs solid phase mixing of alumina and aluminum nitride, which is a simple pathway of synthesis, however, the synthesis temperature needs to be higher than 1750° C. In addition, the price of aluminum nitride raw material is high and will lead to increased cost in mass production, and aluminum nitride is prone to hydrolysis reaction with water. The method of carbothermic reduction and nitridation uses alumina and carbon powder as raw materials, and the aluminum oxynitride powder obtained therefrom is high in purity and low in cost, which makes the method suitable for industrial mass production.
When the method of carbothermic reduction and nitridation is applied to prepare the high-purity aluminum oxynitride powder, the carbon powder and the alumina particles cannot be contacted sufficiently and are difficult to be uniformly mixed, since the reactant is a mixture of solids. While the mass production is carried out and the buildup height thereof is increased, it is not easy for the nitrogen gas to access and perform the nitridation reaction, resulting in the issue of incomplete reaction. The aluminum oxynitride powder synthesized by the method of carbothermic reduction and nitridation is grayish-white or grayish-black, because the aluminum oxynitride (AlON) powder is in a thermodynamically unstable state at a temperature below 1650° C. Therefore, the synthesis temperature is required to be higher than 1650° C., and even between 1750° C.-1825° C. to force the aluminum oxynitride powder of pure phase to be produced. However, at this temperature, it is easy to cause local sintering and agglomeration of the adjacent alumina powder. Conventional method of carbothermic reduction and nitridation has a high reaction temperature and a long reaction time, which will make the particle size of the powder become larger, and make it difficult to obtain aluminum oxynitride powder of a small particle size.
The synthesis of aluminum oxynitride powder feedstocks has a significant effect on the transparency and optical properties of the aluminum oxynitride ceramics. It is needed to use the aluminum oxynitride powder of high purity, uniform distribution, and small particle size as the raw material to facilitate the improvement of the degree of denseness for aluminum oxynitride green and the densification by sintering, and then forming the aluminum oxynitride ceramic having ideal transparency and optical properties. As described in China Patent Publication No. 105837222, the aluminum oxynitride powder of single phase was obtained by employing the sol-gel method. The sol-gel method used aluminum isopropoxide and nano-carbon black as the raw materials, and adds a stabilizer (tetrahydrofuran) mixed with a dispersant (PEG) to obtain a precursor through hydrolysis. The precursor was allowed to stand for 20 hours, dried, milled, sieved, and then subjected to carbothermic reduction at 1650° C. to 1700° C. for 2 hours to obtain the aluminum oxynitride powder of single phase having a uniform distribution of particle size and a particle size of less than 3 μm. However, in China Patent Publication No. 105837222, the preparation of the colloidal precursor was complicated in process and time consuming. As to China Patent Publication No. 101928145, the aluminum oxynitride powder of single phase was prepared using highly active γ-Al2O3 and carbon source (carbon black and nano bamboo charcoal powder) feedstocks by wet high-energy ball milling in combination with the method of carbothermic reduction and nitridation. However, the aluminum oxynitride powder obtained in China Patent Publication No. 101928145 aggregated seriously, and needed long time of wet high-energy ball milling to become aluminum oxynitride powder having a uniform distribution of small particle size (less than 1 μm). China Patent Publication No. 105622104 formed a slurry by ball milling, mixing, and dispersing the activated carbon powder having a particle size of 10 to 100 nm with γ-Al2O3 and a dispersing agent in a pure water for 10 to 36 hours. Subsequently, the mixed powder was obtained by freeze-drying and slow temperature rise for 10 hours. Afterwards, aluminum oxynitride powder having a particle size of less than 2 μm was obtained by rising the temperature to 1700° C. to 1800° C. in a nitrogen atmosphere and keeping for 1 to 2 hours. However, a longer drying time was needed due to the slow sublimation of water at low temperature of the freeze-drying technology. China Patent Publication No. 102180675 utilized aluminum nitrate, urea and nano-carbon black as raw materials, PEG as the dispersant, and ammonium bicarbonate and aqueous ammonia as the precipitant to prepare a precursor by coprecipitation, and then the precursor was precipitated for 24 hours, washed, oven dried, milled, and kept at 1750° C. for 2-4 hours in nitrogen to obtain pure phase aluminum oxynitride, of which the particle size was less than 4 μm. However, China Patent Publication No. 102180675 prepared a precursor by using a coprecipitation method, which is long in precipitation time and cumbersome in process. The method for preparing aluminum oxynitride described above needed a mixing and drying time of up to 10 to 20 hours, resulting in difficult cost control, and therefore cannot be realized in industrial mass production.
Therefore, the industry needs a method for preparing spherical aluminum oxynitride powder, which can employ low cost alumina and carbonaceous material as raw materials, with the use of a simple, energy saving and time-saving process to produce spherical aluminum nitride powder that meets the industrial needs.